I’m reading a book about Thomas Jefferson right now. Well, it’s a semi-fictional account of his daughter’s life, but it’s well-researched and contains parts of his letters. Starting in the 1780s, he became an early adopter of vaccines for smallpox (天然痘). The procedure as described in the book is primitive, but it appeared to work. Here’s a letter he wrote in 1801 describing the experiments he did, vaccinating some of his slaves (yes, it was that era) and even his children.